1. Field of the Invention
The present invention relates to a method for producing a tapered waveguide on a substrate. In particular, the present invention relates to a method for producing a tapered waveguide used in an optical integrated circuit device.
2. Description of the Related Art
Recently, optical parts have been more and more miniaturized and integrated. Consequently, technologies for producing such optical parts in the order of a sub-micron have been established. For example, in the normal formation of a thin film or etching for producing such optical parts, the thickness of a film or the depth of the etching is generally made uniform (processing on a face parallel to a substrate) by keeping the speed as constant as possible. However, in some cases, a tapered structure is obtained by intentionally varying the speed. In particular, a tapered region (a tapered waveguide) in an optical waveguide is an important structure used for bending light in the thickness direction and making the light travel across the boundary of regions having different effective refractive indices to each other without any loss of light.
FIG. 5 shows a side view of an optical integrated circuit device 20 used in a conventional optical information recording/reproducing apparatus (Japanese Laid-Open Patent Publication No. 4-289531). An optical waveguide layer 23 formed on a buffer layer 22 which is disposed on a substrate 21 is optically connected to a photodetector 24 formed in the substrate 21. A first gap layer 25 having a lower refractive index than that of the optical waveguide layer 23 is formed on the optical waveguide layer 23. A second gap layer 26 having an opening 27 and a lower refractive index than that of the optical waveguide layer 23 is formed on the first gap layer 25. An adhesive layer 28 filling the opening 27 of the second gap layer 26 is optically connected to the first gap layer 25. A prism 29 is bonded and fixed onto the adhesive layer 28. The prism 29 is made of a dielectric material having a higher refractive index than that of the optical waveguide layer 23. Light emitted from a semiconductor laser 16 travels through a collimator lens 17 and an objective lens 18 to be radiated onto an optical disk 19. The light reflected at the optical disk 19 enters a photo-detecting element 20 from the opening 27, and then is detected by the photodetector 24. The second gap layer 26 has a tapered region 26a.
FIG. 6 shows another optical waveguide having a tapered region (Japanese Laid-Open Patent Publication No. 4-55802). A light-receiving element 45 is formed in a substrate 41. A buffer layer 42 having a tapered region 44 which partially overlaps the light-receiving element 45 is formed on the substrate 41. An optical waveguide 43 is formed on the buffer layer 42 and the light-receiving element 45. Light propagates through the optical waveguide 43 and enters the light-receiving element 45.
Next, a method for producing these tapered regions will be described. The following methods are considered to be applicable for forming the tapered regions. Part of a substrate on which a film having a tapered region is formed is shielded with a shielding structure and a dielectric film is deposited on the substrate by using a known sputtering method, a vapor deposition method, and a CVD method, thereby forming the tapered region. Alternatively, a tapered region is formed by dry etching, wet etching, ion milling, cutting, grinding, oxidation and the like. Among them, examples of application of a shadow mask method and an etching method will be shown with figures.
Referring to FIG. 7, the shadow mask sputtering method (Journal of Lightwave Technology, Vol. 8, No. 4, pp. 587 to 593, April 1990) will be explained. A mask 61 made of metal is located a certain distance away from an Si substrate 63 by a spacer 62 made of silicon. When the sputtering is performed from the upper part under such a state, the thickness of a film is partially varied since the sputtered particles arrive at the portion shaded by the mask 61. As a result, a portion 64 having a tapered structure is formed as shown in FIG. 7. The shape of the tapered region depends on the cross-sectional shape of the mask, the distance between the mask and the substrate, the size of a target, the distance from the substrate, and the like. The shape of the tapered region is basically determined by the dim shade created by the mask. The reason why the dimness of the shade is caused is that some of sputtered particles are oriented downward and some of them are oriented obliquely.
FIGS. 8A to 8I show a fabrication process for a tapered region. After a first thermal oxidation (silicon oxide) film 52 is formed on an Si substrate 51, a second silicon oxide film 53 capable of controlling the speed of etching is formed on the first thermal oxidation film 52. A photoresist pattern 54 is formed on the second silicon oxide film 53 (FIGS. 8B to 8E). Since the etching speed in the second silicon oxide film 53 is higher than that in the first thermal oxidation film 52 serving as a buffer layer, the etching advances inside the photoresist pattern 54. However, since the etching speed is relatively low in the first thermal oxidation film 52, the first thermal oxidation film 52 is etched little by little in proportion to the time period where the first thermal oxidation film 52 is in contact with etchant. Therefore, the side of the first thermal oxidation film 52 in contact with the photoresist pattern 54 is more etched, and the side of the buffer layer 52 in contact with the substrate 51 is less etched, resulting in a tapered region 55 shown in FIG. 8I. Consequently, an optical waveguide is formed on the first thermal oxidation film 52 having the resultant tapered region 55 (Japanese Laid-Open Patent Publication No. 4-55802).
However, since the tapered region obtained by the above shadow mask method has the thickness of the mask and the height of the spacer of about 1 mm, respectively, a thin film with a tapered region of several mm in length is obtained. Such a long tapered region prevents the device from being miniaturized and integrated. Operations such as application, removal, and washing of a shadow mask make the realization of mass production difficult.
On the other hand, although a taper having a relatively short length can be obtained by wet etching, the etched surface is deteriorated from the time when the film is formed, the surface loses its smoothness and is roughened. This results in optical loss.